Work during the last year has demonstrated that the loss of muscarinic receptors after exposure to agonists is biphasic with a 26 percent loss of receptors during the first minute of exposure and a 43 percent loss of receptors over the next 2.5 hours. The rapid phase is accompanied by a 6 fold decrease in affinity for agonist. Guanine nucleotides cause a similar 6 fold decrease in affinity for agonist and the recovery of the 26 percent of receptors lost during the rapid phase. A class of high affinity muscarinic receptors in heart cell cultures accounts for greater than 20 percent of total receptors. The high affinity receptors are lost during brief exposure to agonist or guanine nucleotides. This data support the presence of a guanine nucleotide regulatory site associated with the muscarinic receptor and suggest a role for a class of high affinity receptors in modulating muscarinic receptor activity.